1. Field of the Invention
This invention relates to optical systems of which the focal length is changed by attaching or detaching a fill-in lens in between the main photo-taking lens and the image plane and more particularly to compact and highly correct fill-in lenses.
2. Description of the Prior Art
It is generally known in the art to elongate or shorten the focal length of a photographic objective by attaching a converter lens in front or in rear of the photographic objective in such a manner as to maintain a fixed focal plane. In this case, the positioning on the object side of the photographic objective, or the front converter lens type requires a bulky converter lens and particularly a front lens element of unduly large diameter, and is difficult to reduce this. Unlike this, the rear converter lens type which arranges the converter lens on the image side of the photographic objective is more advantageous in shortening the lens diameter than the above-mentioned front converter type.
A lens system which may be considered to have taken this kind of advantage is exemplified in Japanese Laid Open Patent No. Sho 54-97423 where use is made of a 6-component converter lens, and, in order to change the focal length of the main photo-taking lens, it is required that the main photo-taking lens is moved forward relative to the camera body, thus elongating the void which later accommodates the converter lens.
On the other hand, the present applicant has proposed in U.S. patent application Ser. No. 219,977 that the number of lens elements constituting the converter lens is reduced so that after the converter lens has been put in between the main lens and the camera body, the accommodating space is shortened in the longitudinal length, thereby giving an additional advantage in that a further minimization of the bulk and size of the camera body is facilitated.
Another aspect of the camera having this kind of converter lens (fill-in lens) incorporating provision is that the diaphragm mechanism and the shutter mechanism are positioned in a space between the main photo-taking lens and the fill-in lens. Therefore, the wider the separation between both lenses, the more advantageously these mechanisms are assembled, and the interference of their lens mountings is avoided. But, this gives rise to a problem wherein as the distance from the main photo-taking lens to the film plane increases, the bulk and size of the complete camera is increased. Then, if a compromise between the requirements of widening the axial separation between both lenses and of providing for a shortening of the physical length of the entire lens system is to be made, the negative refractive power of the fill-in lens has to be strengthened which causes the Petzval sum to be increased in the negative sense. Therefore, as the meridional image surface and sagittal image surface incline toward a negative direction, the curvature of field is increased, and strong outward coma is produced.
As additional examples of the arrangement of the fill-in lens to the rear of the main photo-taking lens, reference may be made to U.S. Pat. Nos. 3,741,621 and 4,206,971.